1. Field of the Invention
This invention relates generally to an arrangement for an offshore platform for drilling or workover operations, production and/or storage operations and in particular to an arrangement for coupling subsea risers to a floating offshore structure which substantially isolates the structure""s heave, pitch, and roll motions from the risers. The term floating offshore structure in this specification includes SPARS, FPSO""s, floating offshore drilling platforms and the like.
2. Description of the Prior Art
The prior art has sought arrangements for coupling subsea risers to floating offshore structures. For example, U.S. Pat. No. 4,606,673 discloses a riser support can for a SPAR Buoy where a single buoyant riser support can supports several risers. The SPAR Buoy, a floating deepwater production and oil storage vessel, includes a riser system whereby risers are connected to a riser float chamber that moves along guides within a vertical passageway within the vessel. The riser support includes an adjustable support which repartitions the load on the risers to assume that each riser is uniformly tensioned. Hull heave motion is decoupled from the riser, but pitch and roll motions of the hull are transferred to the risers. As a result, the risers of this configuration are subjected to cyclical bending. Furthermore, as the adjustable riser supports do not provide any capability of axial vertical flexure relative to one another, this arrangement of riser support does not permit individual riser length fluctuations commonly occurring as a result of operating riser temperature and internal pressure changes. The risers supported as shown therein are subject to cyclical variation in tension as well. The vessel does not have a moonpool and is not designed for drilling or extensive workover operations. If drilling equipment is desired on the SPAR Buoy of the U.S. Pat. No. 4,606,673 structure, the draw works of such drilling equipment would be mounted on the surrounding hull and therefore would require heave compensation.
U.S. Pat. No. 4,966,495 discloses a floating drilling and production structure that includes two independently floating bodies. An outer production and drilling semi-submersible vessel completely surrounds an independently floating wellhead support buoy and supports the weight of the drilling platform, machinery, etc. and is ballasted and anchored in a manner similar to a conventional semi-submersible vessel. An inner constant tension buoy supports many risers. Pitching of the outer vessel is decoupled from the single buoy which supports the risers. The inner buoy (or riser can) is centered within the hull by an annular bumper. The risers are attached to the single riser can by lockdown screws. A significant disadvantage of the U.S. Pat. No. 4,966,495 arrangement is that the semi-submersible xe2x80x9couterxe2x80x9d vessel has a large waterplane area thereby producing large forces in tension due to wave action and hull extension. Another disadvantage is that the drilling or workover equipment is mounted on the outer vessel which induces bending in the drill pipe when there is relative pitch between the outer body and the inner riser support buoy. Thus, the advantage of decoupling the pitch of outer hull from the inner riser support body can only be accomplished when the drilling rig is not in use. Another disadvantage of the arrangement of U.S. Pat. No. 4,966,495 is that individual riser elongation due to temperature and/or internal pressure variation is not allowed for. Furthermore any pitch of the inner riser support buoy results in fluctuation in riser tension because of the large waterplane area of the inner support buoy.
U.S. Pat. No. 4,913,238 discloses a TLP moored riser support module with a conventionally moored semi-submersible hull. A drilling draw works is located on the semi-submersible hull. A relatively small tension leg platform provides a heave-restrained deck for surface wellhead equipment. The hull is free to pitch, roll and heave independently of the risers. The riser support module, being installed within a semi-submersible is exposed to the environment and suffers loading induced thereby. As the semi-submersible hull heave, pitch and roll motions are decoupled from the motions of the riser support module, and the draw works are installed on the semi-submersible hull, the draw works require heave compensation and riser bending due to semi-submersible hull pitch and roll is inherent with this design.
U.S. Pat. No. 4,735,267 discloses a floatation buoy with ballast for supporting multiple risers. The buoy is arranged to be pulled within a moonpool of a buoyant hull. The buoy allows angular flexing of the risers. Individual riser length adjustment is accounted for by allowing the risers to take a catenary shape. The buoy is rigidly connected to the hull of the production vessel. Hanging the risers from the top of the buoy results in static instability, because as the buoy is pulled into the buoyant hull, it becomes unstable and tends to invert unless the buoy is ballasted to negative buoyancy. Because the floatation buoy is not tethered vertically to the sea floor, it is free to heave with the floating production platform, suffering the motions and loads induced thereby.
International patent publication WO 00/58598 shows a riser guide frame which is retractable in the vertical direction for one or more risers on a semi-submersible production vessel. The guide frame provides lateral support for individual riser support buoys. The arrangement of the WO 00/58598 publication provides for lowering the riser support buoys to a point below the splash zone with only the tops of the risers protruding through the splash zone. The riser frame is not tethered to the sea floor, does not have buoyancy, and is rigidly connected to the semi-submersible hull during operation, so the riser frame induces wear through its contact with the risers and their main buoyancy members due to semi-submersible heave. Bending is induced into the risers due to semi-submersible pitch, roll displacements and surge and sway excursions.
U.S. Pat. No. 3,601,075 discloses a system for riser support and guidance within a weathervaning hull. A guide decouples hull heave from riser tension by guiding the riser within a sleeve having rollers with horizontal axes. The system is pendular and allows angular deflection of the riser upon hull excursion through rotation on a spherical bearing or gimbals. The riser includes a buoyant element, but tensioning is accomplished by a hydraulic draw works. Mechanical means maintain the tower and draw works in a vertical position, and the guides act directly on the riser rather than on the riser buoy. The buoy allows bending to occur in the riser, because the buoy is not guided within a framework, so the riser bends when the riser is not vertical.
French patent publication 2,574,367 shows a variety of drilling production and storage platforms which include a central TLP moored-core buoyant structure surrounded by a hull capable of production and storage. The surrounding hull is free to heave up and down on the Tension Leg Pylon or free to heave and rotate on the Tension Leg Pylon or constrained by its own Tension Leg Moorings. Drilling rig and production equipment are disclosed as being placed on the TLP core. The French patent discloses a floating platform with tension leg means for station keeping.
U.S. Pat. No. 6,161,620 shows a riser can which accepts sliding on the surface of the can, rather than on a riser stem.
U.S. Patent Publication 6,176,646 B1 shows a riser arranged pendularly within the riser can. The riser can has an open bottom and an arrangement which allows riser flexing without over bending at the bottom of the riser can through supports which guide the riser, thereby limiting its minimum bend radius due to spar pitch, roll, surge and/or sway.
U.S. Pat. No. 4,702,321 shows a spar with individual flotation buoys attached for tensioning the top ends of each individual riser connected to the sea floor. The patent shows guides for handling the relative motion between the floating structure and each sea floor fixed riser. Stems above and/or below the buoys are described which cooperate with penetrations in the decks to control the relative position of the riser axis while suffering the relative motion of the floating structure.
Because the guides are connected directly to the platform hull, any hull pitch, roll, surge or sway motion is directly transferred to the risers through those guides. Furthermore, all heave motion of the hull is taken at the interface between the hull and the riser stems.
Prior art buoyancy cans for risers are also known that have flatbars welded to their sides which may be designed as sacrificial members to protect the integrity of the buoyancy cans due to their inherent obligation to withstand all relative motion at that interface.
A primary object of the invention is to provide an improved arrangement for decoupling heave, pitch, and roll motions between a floating offshore platform and risers. The object is to provide an arrangement for supporting subsea risers which is applicable to semi-submersible, SPAR, TLP and FPSO platforms and can be installed within a moonpool or turret thereof.
Another object of the invention is to provide a riser support arrangement for a floating offshore platform that provides pendular support between the risers and a surrounding hull, to allow the risers to tilt in a pendular manner in response to lower frequency surge and sway excursion motions.
Another object of the invention is to provide individual riser buoyancy modules installed in a floating framework which is attached to the sea floor through either a drilling riser or a tendon with a drilling rig installed on the floating framework.
Another object of the invention is to provide an arrangement for centering a floating framework within a centerwell of the platform which includes link arms between the floating framework and the platform.
Another object of the invention is to provide a floating framework and platform arrangement where flotation elements of the framework are completely submerged so that no waterplane area exists in order to exert a constant buoyant force on the framework.
Another object of the invention is to provide a floating framework and a platform hull arrangement that provides individual riser buoyancy, a draw works decoupled from hull motion of the platform, and decoupling of hull motion from the risers so as to eliminate cyclical bending of the risers.
Another object of the invention is to provide a floating framework and a platform hull arrangement characterized by decoupling of the risers from hull pitch and constant riser tension regardless of hull motion, thereby avoiding cyclical tension of the risers.
Another object of the invention is to provide individual riser floating framework that is centered within a platform hull arrangement where the floating framework does not have significant variation in tension due to wave action and hull excursion.
Another object of the invention is to provide a floating framework within a platform hull where a drilling rig is mounted to the floating framework so that it does not require heave compensation and does not induce bending due to the elimination of relative pitch between the surrounding hull and riser support buoy.
Another object of the invention is to provide a floating framework within a platform hull where the floating framework with riser support buoyancy modules is completely submerged, with the result that tension load fluctuations are minimized.
Another object of the invention is to provide a floating framework within a platform hull where protection is provided to the riser support module, and the draw works of a drilling rig is mounted on that module to decouple heave, pitch and roll from the risers and the module supports each riser through individual buoyancy devices.
Another object of the invention is to provide a floating framework within a platform hull where vertical risers are supported from the framework with allowance for individual expansion and angularity as a bundle, where risers are decoupled from hull heave and pitch, where the draw works is mounted on the protective guide frame, and where a tendon is moveable to maintain a constant height of the framework above the sea floor.
Another object of the invention is to provide a floating frame within a platform hull where the frame is buoyant and tethered to the sea floor, has a draw works mounted on it, provides pendular coupling between frame and hull so as to avoid inducing bending of risers carried by the frame and positions the risers within a central vertical opening of the protective hull.
Another object of the invention is to provide a floating frame pendularly coupled to a platform hull with guided buoyancy dividers within the frame for tensioning of multiple risers, and with a drilling rig mounted on the floating frame.
Another object of the invention is to provide a floating frame within a platform hull with an arrangement which allows for independent variation in riser length, with process equipment mounted on the hull and with a buoyant frame tethered to the sea floor by a tendon, but with station keeping of the arrangement accomplished with conventional mooring of the hull.
Another object of the invention is to provide a floating frame within a platform hull with a riser can arrangement pendularly coupled to the hull.
A floating offshore arrangement substantially decouples pitch, roll and heave motions between an outer hull structure and buoyantly supported risers which are vertically oriented by a frame or support buoy positioned within the interior of the hull structure. The risers are arranged and designed to slide vertically with respect to the support buoy. The support buoy is coupled to the outer hull structure by a mechanism that allows it to remain in a nearly vertical orientation at a fixed distance above the sea floor while the outer hull is free to heave, roll and pitch. The support buoy is allowed to rotate in a pendular fashion in response to the angularity of risers produced by outer hull excursions in surge and sway.